1. Field of the Invention
The present invention relates to an optical waveguide type photodiode having a plurality of semiconductor layers which are formed one upon another on a semiconductor substrate and which include an optical absorption layer sandwiched between a pair of optical confinement layers for guiding light in parallel with the semiconductor layers. More particularly, the present invention relates to a photodiode which is quick in responsivity, ensures low modulation distortion with respect to incident light and thus is suited for large-capacity optical communications, and to a process of producing such a photodiode.
2. Description of the Related Art
In the field of optical communications using a long wavelength region, pin photodiodes of surface-incident type are widely used. The surface-incident photodiode absorbs light at the surface of a semiconductor layer (optical absorption layer), so that many photo carriers are locally generated in the vicinity of the surface of the optical absorption layer. With the surface-incident photodiode, therefore, modulation distortion with respect to incident light is liable to occur. Also, in surface-incident photodiodes in general, GaInAs semiconductor is used for the optical absorption layer. Since GaInAs semiconductor has a large absorption coefficient with respect to light with a wavelength of 1.3 .mu.m or 1.55 .mu.m, the modulation distortion mentioned above poses a serious problem.
Optical waveguide type photodiodes, on the other hand, have a basic device structure shown in FIG. 1, wherein a plurality of semiconductor layers including an n-type cladding layer 2, an n-type optical confinement layer 3, an optical absorption layer 4, a p-type optical confinement layer 5, and p-type cladding layer 6 are successively formed on a emiconductor substrate 1. The optical confinement layers 3 and 5 are each made of a conductive semiconductor having a refractive index lower than that of the optical absorption layer 4.
In the optical waveguide type photodiode with the above device structure, as light incident on the front facet of the semiconductor layers is guided in parallel with these layers, its light energy is absorbed by the optical absorption layer 4, and carriers excited by the thus-absorbed energy are detected as a photocurrent. Compared with the surface-incident type photodiode, the optical waveguide type photodiode has a thinner optical absorption layer 4, and thus the transit time of photo carriers is short, permitting high-speed operation.
Generally, the thickness (film thickness) of the optical absorption layer 4 of the optical waveguide type photodiode is set to 2 .mu.m or more in order to enhance the light sensitivity. Accordingly, light introduced into the optical absorption layer 4 is absorbed in large part in a region near the front facet while being guided for only a relatively short distance. As a result, a large number of photo carriers are generated locally in the vicinity of the front facet of the optical absorption layer 4, and such local generation of photo carriers causes modulation distortion.